In telecommunications, a femtocell, originally known as an Access Point Base Station, is a small cellular base station, typically designed for use in residential or small business environments. The femtocell is a user-deployed home base station (BS) that provides improved home coverage and increases the capacity for user traffic using a backhaul connection to a service provider, such as an Internet Protocol (IP) connection over the user's Digital Subscriber Line (DSL), cable, satellite, fiber-optic, or other high-speed or broadband connection. Current femtocell designs typically support 2 to 4 active mobile phones in a residential setting. Due to co-channel or adjacent-channel operation, it is very challenging to address interference between nearby femtocells or between femtocells and an existing macrocell, that is, a cell in a mobile phone network that provides radio coverage served by a power cellular base station (tower).
The prior art and currently proposed solutions for minimizing inference between femtocells and femtocells or between femtocells and macrocells can be classified into three categories: (1) power control; (2) separation in frequency; and (3) separation in time.
Power Control. A power adaptation algorithm can mitigate interference from femtocells to macrocells. The basic theory is to lower the transmission power of a femtocell BS and femtocell user equipment (UE) as much as possible, so that the interference to the macrocells is reduced while maintaining reasonable performance of femtocells. To achieve this, the femtocell utilizes the measurement of the channel parameter from its attached UEs to set the transmission power of both the femtocell BS and femtocell UEs.
Separation in Frequency. The basic theory is to separate the transmission of femtocells and macrocells in different frequencies. This reduces interference by limiting the frequencies that are shared by femtocells and macrocells.
Separation in Time. Another interference mitigation technique is time re-use or time sharing, that is, separating the transmission of different femtocells, or femtocells and macrocell UEs, in time. This interference mitigation technique has been proposed for Universal Mobile Telecommunications System (UMTS) femtocell systems. The idea is to lower (or even turn off) the femtocell downlink (DL) transmit power during certain time periods to reduce interference to close-by UEs (either femtocell UEs attached to another femtocell BS or macrocell UEs). This technique assumes a square ON/OFF power pattern for the femtocell. The period and utility cycle of the patterns are design parameters. Given a certain pattern, each femtocell can pick the best time offset for its UE by sensing the time offset of the neighboring femtocells. This could reduce the interference between a femtocell and other femtocells or between femtocells and macrocells.